Chapter 13 Structure Determination by Spectroscopy I IR

Chapter 13 Structure Determination by Spectroscopy (I): IR “study of interaction of light with molecules” r electromagnetic E = hn = hc/l; n =frequency, l=wavelength excitation in quantized orbitals, 502 Figure 13. 1 spectrum: amount vs n of light, 504 Figure 13. 2 spectrometer: a scheme, 503 middle electromagnetic Org. Chem-Chap 13 radiation light spectrum: 505 Figure 13. 3 1

v IR Spectroscopy: Functional Group r. E of vibrational levels E of IR radiation vibrational functional n group identification: 507 Figure 13. 4 k / M; k=force constant, M=reduced mass mthe stronger the bonding, the higher the wavenumber mhigher stretching wavenumber than the bending mhigher wavenumber for a light hydrogen atom: 508 top mthe Org. Chem-Chap 13 modes: stretch & bend 506 top more polar the bonds, the stronger the absorption: %T 2

v Hydrogen Region: 3600~2700 cm-1 r bond strength: C-H < N-H < O-H C-H: H-Csp 3 (3000~2850 cm-1) < H-Csp 2 (3100~ 3000) [H-C(O): 2830~2700] < H-Csp (ca. 3300); 509~511, Figure 13. 5 - 8 O-H: hydrogen bonding; 3550 ~ 3200 cm-1 (br & s) 512, Figure 13. 9 CO 2 H: N-H: very br & s (3000 cm-1); 513, Fig. 13. 10 w; 3400~3250, 513~4, Figure 13. 11~12 practice: Org. Chem-Chap 13 514, Problem 13. 5 3

v Triple-Bond Region: 2300~2100 cm-1 r C C: w, 2150~2100 cm-1, 510, Figure 13. 7 r C N: m, 2260~2220 cm-1, 515, Figure 13. 13 r CO: s, 2170 cm-1, CO 2: s, 2400~2300 cm-1, 516~7, Focus On practice: Org. Chem-Chap 13 515, Problem 13. 6 4

v Double-Bond Region: 1900~1500 cm-1 r C=O: typical; s, 1715 cm-1, 518 Table 13. 1 conjugation ring-size effect: ↓ (20~40 cm-1), 518 top effect: 1715 (6 -ring) < 1751 (5) < 1775 (4) r C=N: m, 1689~1471 cm-1 r C=C: w, aliphatic 1660~1640 cm-1 aromatic: r NO 2: Org. Chem-Chap 13 1600, 1580, 1500 & 1450 cm-1 s, 1550 & 1380 cm-1, 521 Fig. 13. 14 5

v Fingerprint Region: < 1500 cm-1 r complex but identification of unknowns single bond stretching & bending overlapped r C-O: s, 1300~1000 cm-1; ester & ether C-N: r Ar-H: m, 1250~1000 (Ar-N: s, 1340~1260) s, 900~675 cm-1 (bending) arenes: Org. Chem-Chap 13 1600~1450 & 3100~3000 cm-1 6

v Interpretation of IR spectra r characteristic R-H: R=R: 3600~2700 cm-1 R R: 1800~1500 cm-1 practice: r Always n: 522~3 Table 13. 2~3 2300~2100 cm-1 C-O, C-N, Ar-H, etc. 523~9, Figure 13. 15~13. 22 double-check your assignment !!! practice: 519, 530 Prob. 13. 8 -9, Prob. 13. 10 -11 & 532 Problem 13. 12 Org. Chem-Chap 13 7